<!doctype html> 
<html> 
<head> 
    <meta charset="utf-8"> 
    <title></title> 
    <style type="text/css">
*{margin: 0;padding: 0;}

    </style>
</head>
<body>
<script type="text/javascript">
	
this.b = 10;
json = {
	a:function(){
		this.b = 20;
	},
	d:function(){
		c.call(this.f);
		this.f = 10;
		alert(this.f+this.b);
	},
	f:5
}
//json.b = 20; json.f = 10; 30 return undefined
function c(){
	return this;
};
json.a();
alert(json.d());
//30 x undefined


</script>
</body>
</html>